Full disclaimer: I'm the kind of guy who will take a near-truckload of lighting gear to a studio shoot, and an armload to the zoo with family. Speedlights, to me, just aren't enough. Outdoors, especially against sunlight, you're going to end up against what I call the sync speed curve: ISO 100, 1/250th, and now you're stuck living at f/5.6 or f/9 depending on conditions. Once you have to stop down, you're throwing away some of your speedy light, and now you have to cheat them closer and closer to the subject to have any hope of them doing what you want, especially if you're shooting through ANY modifier (even a 1/4 CTO gel costs you 1/3 stop). HSS lets you open up, but my non-scientific research suggests that as soon as you transition to HSS, you've given up 1-2 stops more, so HSS is only an option if you were ahead of the curve already but you want thin DoF.

If it were me, I'd take a good, hard look at (renting or buying) a Profoto B2 kit. A one-light kit means you could put the sun behind and fill the front, or put the sun in front on one side and fill on the other. A Zoom Reflector is relatively compact and very powerful. An OCF 1x3 softbox is quite lightweight and will fit in a suitcase just fine. You can get a $6 brass thingy to screw on top of a monopod (if you'd have a holder) or a tripod to use as an impromptu stand. The B2 is 4-5x the power of a 580EXII, with better recycle time, and being a separate pack/head kit, helps keep the center of gravity lower. It can do HSS so you can do thin DoF if you wish, but it also has the power so you can keep it comfortably back out of the frame.

Not a direct comparison reference shot, but at least a worthy example of what these can do. I shot team photos for a charity walk in April, and had three lights: a Profoto B1 to my far left, with a Zoom Reflector, another B1 just barely to my left, with a Widezoom Reflector (smoother coverage, but big enough that you can't put it in a suitcase), and a B2 to my right, with a Magnum Reflector (gains a stop on the Zoom Reflector). Yes, it was three lights, but they were all about a half-stop under their max (in the hopes of making the batteries last, which they did far better than I expected) and I kept them back far enough that I could easily move one large group out of the way to my right while another group came in from my left and they wouldn't be a major trip hazard. See http://photos.templin.org/images/TeamBlitz-101.jpg

Also, I was a guest at a family wedding and brought a "lightweight kit", in this case a B1 (one stop more power than a B2, but all the weight in one place so it can be top-heavy) and my 1x3 softbox. I shot available light during the ceremony, then added the light when they drove the green pickup over to the reception. In order to help out the paid photographer (who was essentially screwed by the toast being so late), I just turned on the modeling light (20W LED, comparable to a 70W incandescent bulb) and took the remote off so I wouldn't fire the B1 during those shots. See https://www.flickr.com/photos/alibea...57657308084120 for what I got.